In electrophotography, a latent image is created on the surface of an insulating, photoconducting material by selectively exposing an area of the surface to light. A difference in electrostatic charge density is created between the areas on the surface exposed and those unexposed to the light. The latent electrostatic image is developed into a visible image by electrostatic toners containing pigment components and thermoplastic components. The toners, which may be liquids or powders, are selectively attracted to the photoconductor's surface, either exposed or unexposed to light, depending upon the relative electrostatic charges on the photoconductor's surface, development electrode, and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles.
A sheet of paper or intermediate transfer medium is given an electrostatic charge opposite that of the toner and then passed close to the photoconductor's surface, pulling the toner from the photoconductor surface onto the paper or intermediate medium still in the pattern of the image developed from the photoconductor surface. A set of fuser rolls or belts, under heat, melts and fixes the toner in the paper, subsequent to direct transfer or indirect transfer when an intermediate transfer medium is used, producing the printed image.
The electrostatic printing process, therefore, comprises an ongoing series of steps in which the photoconductor surface is charged and discharged as the printing takes place. In addition, during the process, various charges are formed on the photoconductor surface, the toner and the paper surface to enable the printing process to take place. Having the appropriate charges in the appropriate places at the appropriate times is what makes the process work.
Contamination of print media arises in electrophotographic printers and copiers as a result of charge accumulation on the fuser hot roll or belt and the associated pressure roll. This contamination results from the offset of toner from the print media onto the contacting fuser hot roll or belt due to unfavorable electrostatic fields in and around the fusing nip (i.e., the nip formed between the fuser roll or belt and the pressure roll). This contamination ("toner offset") results in a printed page of poor quality, generally characterized by the appearance of undesired white lines followed by toner debris after one additional revolution of the fuser hot roll or belt.
Clearly, the toner offset problem is a very important one in electrostatic printing and, accordingly, a number of solutions to it have been proposed, including:
(1) Adding a conductive agent in the form of carbon black or an ionic conductive additive to the release layer coating of the fuser roll (or belt), pressure roll, or both. This results in decreased electrostatic charge accumulation on the fuser member, but also results in some loss of release properties as compared to an unfilled fluoropolymer coating. The underlying conductive layer of the hot roll or belt in those structures is typically grounded. See, for example, U.S. Pat. No. 5,045,891, issued Sep. 3, 1991 (single crystal (carbon and other) fibers in Teflon outer layer of hot roll); U.S. Pat. No. 4,434,355, issued Feb. 28, 1984 (rough primer layer containing conductive fibers penetrates the outer layer roll); and U.S. Pat. No. 4,320,714, issued Mar. 23, 1982 (pressure roll with grounded electroconductive layer beneath insulating PFA or silicone outer layer). PA1 (2) Applying a biased voltage to the conductive hot roll (or belt) substrate or to the pressure roll core, or both. In this case, the roll or belt surface coating needs to have either a short time constant (resistive) or a dielectric breakdown voltage to applied bias to enable the bias voltage to be effective. Underlying layers between the bias electrode and the surface coating also need to provide a current path to the coating layer. The external bias is applied (or allowed to develop triboelectrically, using diodes) in a direction which keeps toner on the print media. This method is utilized, in conjunction with a silicone covered heat roll and a resistive-rubber core, PFA-sleeved pressure roller, in the HP SXIII printer from Canon. PA1 (3) Adding a tribocharging surface-active agent to the pressure roll surface in an attempt to reverse the tendency to accumulate electrostatic charge of the wrong sign on the pressure roll. For instance, a fluoropolymer-sleeved pressure roll tends to tribocharge negative when scrubbed against paper. When used in a system with negative toner, a tribocharging surface-active agent would need to cause the fluoropolymer coating to charge positive, rather than negative. See, for example, U.S. Pat. Nos. 4,616,917 and 4,640,600, issued Oct. 14, 1986. PA1 (4) Using a discharge brush touching or adjacent to the surface of the pressure roll. In this case, a discharge brush introduces wear to the roll surface, and plays an unintended contamination collection role, both of which are undesirable. Charge removal from the thick rubber-covered pressure roll is also ineffective at preventing charge accumulation on the hot roll or belt, consequently not completely solving the electrostatic contamination problem. PA1 first and second rotatable members forming a nip therebetween, which transport from an entrance side through said nip to an exit side a recording material, thereby fixing toner to create an image on said recording material; PA1 means for driving at least one of said first and second rotatable members; and PA1 an electrode located on the exit side of said nip adjacent to the non-image carrying side of said recording material, said electrode providing an electric voltage in proximity to said recording material of a polarity opposite that of the toner. Preferred electrodes (discharge members) are in the form of a brush, a saw tooth structure or a wire, with the saw tooth structure being particularly preferred. The electrodes preferably carry a positive voltage of from about 200 to about 1,000 volts, most preferably from about 400 to about 600 volts.
U.S. patent application Ser. No. 09/393,571, filed Sep. 10, 1999, discloses heat rolls and fuser belts which seek to minimize toner offset while still maintaining excellent release characteristics of the printed page from the fuser. The heat rolls comprise a core member having coated thereon a plurality of concentric layers, wherein at least one of said layers (preferably the top layer) does not contain electrically-conductive materials, and wherein the roll exhibits electrical breakdown at about 250 volts or less.
U.S. Pat. No. 5,722,012, Saitoh, issued Feb. 24, 1998, describes the use of a toner removing device positioned between the image-forming portion and the toner fixing portion of the printer. The device produces an electrical charge opposite the charge of the toner on the printed page, and its purpose is to remove (by electrostatic attraction) toner from the back of the page. There is no discussion about the effect of this device on minimizing toner offset from the front of the page.
U.S. Pat. No. 5,287,153, Senba, issued Feb. 15, 1994, describes a method for minimizing toner offset in a printer. The second embodiment described (see FIG. 4) appears to have conductive needles which contact the paper, removing the positive charge from the paper and applying it to the conductive base of the fixing roller. This embodiment does not apply a biasing voltage to the paper.
U.S. Pat. No. 4,525,058, Hirabayashi, et al., issued Jun. 25, 1985, describes a charge removing means which is located at the exit side of the fixing rollers in a printer. The charge removing means consist of conductive needles which are placed near (but not in contact with) the printed sheet. The needles are grounded and act to remove any charge from the paper. No biasing voltage is applied to the paper.
U.S. Pat. No. 5,253,024, Okuda, et al., issued Oct. 12, 1993, describes a charge removing brush which contacts the backside of the printed page, removing accumulated charge via grounding (see FIG. 8). There is no charge directly applied to the printed page in this embodiment.
It has now been found that toner offset can be minimized, without compromising the release properties of the fuser roll, by placing an electrode (i.e., a discharge member) adjacent to the fuser nip exit and the back side of the printed page, such that the electrode provides to the page a bias voltage of opposite polarity to the charge of the toner on the page. This approach not only minimizes toner offset, but it does so without requiring reformulation of the fuser hot roll or pressure roll coverings, and, further, without compromising the release properties of the printed page from the fuser roll or pressure roll.